This invention relates to a method of protecting a gas injection system from inactive gas distribution nozzles. It is a common processing step in chemical plants and oil refineries to inject a gas into a vessel containing a liquid or a bed of particles so that the gas flows in an upward direction through the vessel. When the vessel contains a bed of particles, the gas may be injected at a flow rate sufficient to fluidize the particle bed. The gas is distributed across the horizontal cross-section of the vessel by means of a distributor containing a multiplicity of nozzles through which the gas flows out of the distributor and into the vessel.
It is normally essential to the process taking place in the vessel that gas flow across any horizontal cross-section of the vessel be relatively uniform, to ensure that the entire contents of the vessel will be contacted by the gas. This is accomplished by locating the nozzles in a relatively uniform pattern across a horizontal cross-section of the vessel and properly designing the distributor, taking into account the required gas flow. The distributor often comprises a piping network, which is sized and configured so that the pressure drop from the network inlet point to and including any nozzle is approximately equal to that to and including any other nozzle; thus there is an approximately equal flow out of each nozzle. Since the lengths of the flow paths from the network inlet point to each nozzle are normally not equal, the design methods used to achieve approximately equal flow through each nozzle are to use varying pipe diameters and to make the pressure drops across the nozzles large in comparison with the pressure drops for the flow paths from the network inlet point to each nozzle. An example of a piping network distributor and information on distributor design may be found in U.S. Pat. No. 4,032,300 (Parker et al.). Another type of distributor comprises a vessel-like construction having a very small height, all surfaces of which are approximately parallel to those of the processing vessel, and which has a multiplicity of gas nozzles on its upper surface.
In many processes, the pressure in the vessel varies considerably, as a function of process changes taking place in processing elements upstream or downstream of the vessel. Often, it is necessary that the flow of gas to the nozzles vary in response to changed process conditions. The temperature of the gas may not remain constant. Any one of these variations in pressure, flow, and temperature affect the pressure drop across the nozzles, as will be seen by means of the equation and text presented herein. If the average pressure drop across the nozzles falls to a low value relative to the static head in the vessel, the quantity of gas flow through each nozzle will vary widely, with some nozzles passing very little gas or no gas. A nozzle having little or no gas flowing through it is called an inactive nozzle. The average pressure drop across the nozzles is the average of the pressures inside the distributor at the inlets to each of the nozzles minus the pressure in the vessel in the vicinity of the nozzles. As mentioned above, an effort is made during design of the distributor to minimize the individual variations from the average. One of the results of inactive nozzles is uneven gas distribution in the vessel, causing less efficient operation of the process. In addition, liquids or solids may travel backward through the nozzles into the distributor and possibly further upstream to cause damage to the blower supplying gas to the process. It may be necessary to shut down the process to clean out the distributor and piping, particularly in the case where the vessel contains a bed of solids.
In order to prevent inactive distributor nozzles, it is known to those skilled in the art that the average pressure drop across the nozzles must be no less than a value consisting of a fraction times the static head in the vessel. The fraction generally accepted by those skilled in the art is approximately 0.3 to 0.33. The static head in the vessel is taken in normal operation and is the product of the actual height of the liquid or particles in the vessel and the density of the vessel contents during processing.
It should be noted that the circumstances described above exist even though the pressure of the gas in the distributor exceeds the pressure in the vessel. The vessel contents will flow into the distributor when vessel pressure is greater than that in the distributor; however, it is not the primary object of the instant invention to correct this pressure imbalance, but to prevent the occurrence of inactive nozzles.
This invention is especially adapted for use in connection with a regenerator vessel used in the fluid catalytic cracking of hydrocarbons. In this process, hydrocarbon feed and catalyst are introduced into a reactor in which the cracking reactions take place to produce hydrocarbon products. As a result of the reactions, the catalyst acquires a coating of carbonaceous matter, usually referred to as coke, which interferes with the effectiveness of the catalyst. The normal procedure in a fluid catalytic cracking plant is to continuously withdraw catalyst, treat it to remove coke, and return it to the reactor. Treatment is accomplished by subjecting the catalyst to a high temperature environment in a pressure vessel called a regenerator. The high temperature environment is comprised of pressurized air serving as a fluidizing medium for the catalyst and as a source of oxygen for combustion of the accumulated surface deposits (coke). Fluid catalytic cracking is a very well known process with a multiplicity of variations, including different equipment arrangements and operating conditions. Information on the process is available from numerous sources, including Petroleum Processing Handbook, Bland and Davidson, McGraw-Hill, 1967, page 3-2 et. seq. FIGS. 10-22 and 10-23 of this reference show regenerator vessels in typical configurations with typical control systems.
Air flow to the distributor and nozzles in a regenerator is normally controlled at a constant value corresponding to the hydrocarbon feed rate to the catalytic cracking unit. When the hydrocarbon feed rate is reduced, it is necessary to reduce the air flowing to the regenerator. A reduction in air flow rate reduces pressure drop across the nozzles and will cause inactive nozzles if it is sufficiently large. It is not possible to simply establish a minimum below which the air flow rate will not be reduced because the minimum flow required to prevent inactive nozzles, when flow rate alone is considered, is higher than the lower range of air flow rates at which it is necessary to operate the regenerator. If nozzles become inactive, catalyst which is not fully regenerated may be returned to the reactor, resulting in a less efficient catalytic cracking process. Also, catalyst may flow into the distributor and air piping. The inventor has knowledge of at least one instance in which, during apparently normal operation of a regenerator, catalyst moved through inactive nozzles and through pipelines and valves to the air blower supplying the nozzles, resulting in severe damage to the blower.
Pressure in a regenerator is also a controlled variable which must be adjusted in response to process conditions. When regenerator pressure is increased, pressure drop across the nozzles decreases. This is true even though the flow rate through the nozzles is controlled at a constant value. Temperature of the air supplied to the nozzles, which is not normally controlled, can vary, thereby affecting pressure drop across the nozzles. The variable factors which determine the temperature of the air at the nozzle inlets are temperature of the air drawn from the atmosphere by the blower, heat of compression, and regenerator temperature, which affects the amount of heat transferred to the air as it flows to the nozzles.
Thus, it can be seen that, as a practical matter, the possibility of inactive nozzles developing as process operating parameters change cannot be easily taken into account by the operators of the unit and that a system for automatically monitoring for process conditions that lead to inactive nozzles is desirable.
In a complex processing unit, such as a fluid catalytic cracking plant, hundreds of process variables are continuously monitored and often adjusted by the process operators. Only a small number of these adjustments have a bearing on the occurrence of inactive nozzles. Even though the results of the occurrence of inactive nozzles can be of major consequence, this is not considered to be a major operational problem and therefore receives little attention in the day-to-day operations. To require process operators to consider the problem before making a change and decide whether the change has potential to lead to inactive nozzles and then work through an equation to find if there will be a problem is clearly impracticable.